Magnetic resonance (MR) imaging is commonly used for pre-operative images. Examples of pre-operative image images include medical images acquired pre-operatively or surgical plans based on pre-operative images. Ultrasound (US) is frequently used for diagnostic and intraoperative imaging purposes. An example of intraoperative data includes medical images acquired during surgery in the operating room. It is advantageous to cross-reference pre-operative images with intraoperative images. Unfortunately, features that may be readily apparent in a pre-operative image may be difficult to discern in an intraoperative image. Likewise, features that may be readily apparent in an intraoperative image may be difficult to discern in a pre-operative image.
Image registration is the process of transforming different sets of data into one common coordinate system. The need for intraoperative registration arises because there may be an unknown spatial relationship between pre-operative images and intraoperative data in the operating room. While it may be possible to visualize a portion of the anatomy of a patient within the pre-operative medical images, and also to visualize the same anatomy using intraoperative data such as ultrasound, the precise spatial correspondence between the representations may be unknown. For example, the soft tissue of an internal organ may be displaced or deformed during surgery, making it difficult to correlate the location of a feature in an intraoperative ultrasound image to the location of the feature in a pre-operative magnetic resonance image.
Image processing and analysis involves extracting features, describing shapes and recognizing patterns. Such tasks refer to geometrical concepts such as size, shape, and orientation. Mathematical morphology uses concepts from set theory, geometry and topology to analyze geometrical structures in an image. In the context of image processing, morphology is the name of a specific methodology designed for the analysis of the geometrical structure in an image. Mathematical morphology examines the geometrical structure of an image in order to make certain features apparent, distinguishing meaningful information from irrelevant distortions by reducing the meaningful information to a simplification, or skeletonization. Such a skeleton suffices for feature recognition and can be handled much more economically than the full symbol.
The basic morphological operations, erosion and dilation, may produce contrasting results when applied to either grayscale or binary images. Erosion shrinks image objects while dilation expands them. Dilation generally increases the sizes of objects, filling in holes and broken areas, and connecting areas that are separated by spaces smaller than the size of the structuring element.
The understanding and interpretation of ultrasound images is greatly facilitated by registering the ultrasound images with images generated with other modalities, most notably, magnetic resonance imaging (MRI). As noted above, image registration is the process of transforming different sets of data into one common coordinate system. Such sets of data may be multiple photographs, data from different sensors, from different times, or from different viewpoints. Image registration is used in computer vision, medical imaging, military automatic target recognition, and compiling and analyzing images and data from satellites. Registration is necessary in order to compare or integrate the data obtained from these different measurements.
In imaging technology, a fiduciary marker, or fiducial, is an object used in the field of view of an imaging system which appears in the image produced for use as a point of reference or a measure. A fiducial may be either something placed into or on the imaging subject, or a mark or set of marks in the reticle of an optical instrument. Fiduciary markers are used in a wide range of medical imaging applications. Images of the same subject produced with two different imaging systems may be correlated by placing a fiduciary marker in the area imaged by both systems. In this case, a marker which is visible in the images produced by both imaging modalities must be used.
There are disadvantages of the fiducial marker-based registration approach. First, it may be necessary to affix the fiducial markers to the patient, for example, glued to the scalp or drilled into the skull, before the pre-operative images are acquired. This may be an invasive and costly process which may require an additional surgical procedure. Second, depending upon the type of intraoperative sensor used, the fiducial markers may need to be physically exposed during surgery, even if the markers are located far from the primary surgical site. This may result in additional blood loss and additional time required to suture the soft tissues in the vicinity of the markers. Additionally, there are instances when fiducial based registration of ultrasound images with MRI images is unsuccessful.
Therefore, there is an unmet need for registration of ultrasound images with MRI images with improved visual accuracy and greater likelihood for success than fiducial-based registration of ultrasound images with MRI images, without the costs, risks, and complexities of fiducial based registration.